Many toxins require the normal function of membrane traffic pathways to exert their toxic effects. We recently developed an imaging-based medium-throughput screen with intact cells to identify chemicals that affect different pathways of membrane traffic. From two different screens and using over 19,000 compounds, we identified two chemicals, exo2 and secramine, with unique and specific effects on the function of cholera toxin and anthrax edema toxin. This suggests a potential novel route to therapeutics to mitigate the effects of these toxins, and possibly also those of ricin and shiga toxin, which, like cholera toxin, enter cells through endocytosis and require transport to the ER to exert their action. We now propose studies to determine the molecular basis for the biological activity of secramine on retrograde traffic from the TGN and the ER and to elucidate its molecular target. The results of these studies will ultimately allow us to design or discover a more potent analog of secramine.